The present invention relates to electrical heating devices that use positive temperature coefficient ceramic elements in self-regulating heaters.
Introduction
Production of heating tape is a substantial industry in the United States, with a large existing body of background art. Before developing my prototype for use in the heating system development in connection with the above cited contracts, some of the commercially available tape was purchased. I attempted to use it around areas on composite surfaces, to thermally isolate the areas from ambient so that they could be heated to high and precisely regulated temperature for the purpose of composite repair. Results with the available tape were not acceptable for a variety of reasons:
The most common use for heat tape is to prevent the cooling of an already warm, insulated object. The heat transfer rates that available heating tapes can provide are generally quite limited. The maximum power density found was 72 watts/foot, for an unregulated tape.
An xe2x80x9cunregulatedxe2x80x9d tape is one which delivers heat at a substantially constant rate, based on the applied voltage and its own electrical resistance, regardless of its own temperature. In contrast, a xe2x80x9cself-regulatedxe2x80x9d tape is one which automatically reduces its rate of heating at elevated temperature.
If the tape is truly xe2x80x9cself-regulatingxe2x80x9d, the rate of heat delivery decreases very dramatically at and above a specific threshold temperature. Alternatively, some heating tapes deliver a gradual and continuous reduction in heating rate as temperature increases. Such tapes are better described as xe2x80x9cself-limitingxe2x80x9d; they cannot truly regulate to a specific temperature, but they prevent the tape from heating to a potentially destructively high temperature.
As indicated above, the maximum power density found in a commercially available tape was 72 watts/foot. This was not only too low for our purpose (since our objective was to effectively heat to very high temperature in the presence of heat losses), but the lack of regulation made the tape unsuitable due to potential for destructive overheating.
The heating tapes are invariably electrically insulated. Electrical insulations generally have low thermal conductivity, e.g. on the order of 0.1 Btu/hr-ft-xc2x0F., as compared with about 200 Btu/hr-ft-xc2x0F. for copper. Therefore, the temperature of the surface being heated will be very much lower than the temperature of the heating element or wire, by an unpredictable amount, unless steps are taken specifically to minimize heat transfer resistance.
If a PTC (positive temperature coefficient) heating element is used, the material is generally either a specially prepared polymer or barium titanate ceramic. Thermal conductivity is low in either case, in the range of about 0.1 to 2.0 Btu/hr-ft-xc2x0F. Therefore heat developed throughout the body of the element is partially trapped, and the element interior is much hotter than the element surface.
As a result of thermal resistance and gradients within the element, power density of available self-regulating or self-limiting tape is even lower than for the unregulated components. The maximum power density found in an available self-regulating component was 50 watts/foot. Most commercial tapes are limited to less than 20 watts/foot and, as a practical matter, even 20 watts/foot is only attainable when the surface being heated is very much colder than the tape.
If the component is made literally in the form of a tape, being typically wide but thin, it typically has very little flexibility except in xe2x80x9cverticalxe2x80x9d bending (in the plane perpendicular to its width). It cannot be bent laterally, and it cannot readily be stretched or compressed.
If it is a PTC xe2x80x9ctapexe2x80x9d, which typically is quite thick, it also is fairly rigid to torsion.
As a result, available PTC tapes can be applied to a flat surface or follow the simple contour of a cylinder (a pipe or tank, for example), but cannot follow arbitrary or complex contours to heat a complex structural surface, e.g. for the purpose of composite repair.
Other PTC components, which are designed to address the flexibility issue, do not have the relatively extreme width of xe2x80x9ctapesxe2x80x9d. Width and thickness are essentially identical in such components, and they are called heating xe2x80x9ccablesxe2x80x9d or xe2x80x9cropesxe2x80x9d rather than tapes. As a result, they can follow complex contours, but available contact area for heat transfer is severely reduced. Power density and temperature regulation are inherently further compromised.
These problems were the impetus for the development of a new heat tape. The prototype showed much better power density, heat transfer and temperature regulation than the commercially available tape that had first been tried. However the prototype was handmade and was fragile, with limited flexibility. The tape according to the present invention includes:
1. Available, actually attainable power density up to or even exceeding 120 watts/foot.
2. Thin-film (e.g.≈0.010xe2x80x3 (0.25 mm) or less) electrical insulation, for improved heat transfer.
3. Wide, flat tape geometry.
4. Easy flexibility in stretching and compression, torsion, and both lateral and vertical bending.
The purpose of the invention is to heat a surface to a prescribed temperature, with a compact and inexpensive heater that does not require any external regulator, i.e. is self-regulating.
For example, the invention may be used:
a. to provide a warm ambient temperature boundary around a region of controlled temperature, to facilitate accurate control and temperature uniformity within the region.
b. to heat a region to an elevated temperature with moderate precision, with no other means of regulation required.
c. to maintain elevated temperature, as of a pipe or vessel, without risk of overheating.
In addition, of course, the invention can also be used in the manner of background art self-regulating heaters, to prevent a surface from becoming cool with no risk of overheating in varying conditions.
Existing devices directed to the same objectives include heater tapes, heating ropes, and a variety of special purpose heaters, especially those constructed with self-regulating (positive-temperature-coefficient, or PTC) heating elements.
In general, a PTC resistive heating element is one whose electrical resistance rises significantly with an increase in its temperature, thus limiting its potential power dissipation at constant voltage.
Some examples of relevant background art include U.S. Pat. Nos. 5,937,435; 5,922,233; 4,341,949; 4,324,974; 4,223,208; 4,117,312; 4,072,848; 4,673,801; 4,574,187; 4,425,497; 4,395,623; 4,330,703; 4,177,376; 3,914,727; 3,861,029; 3,749,879; and 3,748,439.
The following performance related problems are associated with the background art:
The regulated temperature is that of the heater, not of the surface being heated, so good heat transfer (low and predictable thermal resistance) between the heater and surface is essential for regulation. However good heat transfer is never achieved in the background art and the tolerance on xe2x80x9cregulatedxe2x80x9d surface temperature is extremely wide.
The heating element must almost always be electrically insulated for safety, and electrical insulators are invariably thermal insulators as well. As a result, there will always be a large and uncertain thermal resistance and xcex94T (temperature drop) within the insulating layer between the heating element and the surface.(Indeed, since PTC elements are generally either polymers or ceramics, not metals, there is often a high differential temperature between the element interior and its own surface.) Adjacent surface temperature will be substantially below heater temperature.
Background art devices offer very low power density. The xe2x80x9chigh powerxe2x80x9d variant or model of a typical commercially available self-limiting heater tape provides a maximum of about 20 watts/foot at normal room temperature, and power density decreases further with increasing temperature.
The devices are bulky and complex structures, relative to the simplicity of the resistive elements that are functionally central. Tapes with polymer elements are generally molded between two buses, but then the further molding on of insulation (applying a layer of insulation) adds bulk as well as thermal resistance. Ceramic elements are generally held in place, relative to conductive contacts, with clamps and springs. They are generally not soldered (not even silver-soldered) to relatively rigid metal members because metals expand much more on heating than ceramics do, and the thermal differential expansion could create destructive tensile stresses in the ceramic.
In some cases, particularly where polymer elements are used, there is no distinct regulation temperature. Instead, the element""s electrical resistance increases gradually and continuously with temperature, which is unsuitable for regulation to a specific temperature within a reasonable tolerance.
The devices lack mechanical flexibility to accommodate surfaces of arbitrary contour to be heated.
Even the polymer-element heater tapes with construction similar to a two-conductor wire, even in a single degree of freedom (up-down bending), have very limited flexibility and cannot follow tight radii and complex surface contours. Available tapes have virtually no flexibility in a second degree of freedom (side-to-side bending and/or torsion).
Heating xe2x80x9cropesxe2x80x9d are also available with flexibility in two degrees of freedom. However the heating ropes have very poor and very little thermal contact, even lower power density, and offer virtually no regulating capability.
Due in part to the no-soldering constraint, available products based on PTC ceramics can primarily prevent a hot surface from cooling, e.g. to prevent freezing or cooling of a process tank or pipe. They function as a kind of powered insulation, but they do not really have the capability to heat a surface and regulate its temperature with any degree of accuracy.
The term xe2x80x9cheat transfer surfacexe2x80x9d is used repeatedly in this specification. It is defined for our purposes as the surface of a heater which is in contact with the surface to be heated, and through which the heat is transferred.
The PTC ceramic (e.g. barium titanate) heating element is soldered directly to heavy copper or other solderable metal contacts/electrodes, which can conduct both heat and electricity with low resistance and present a smooth, extended heat transfer surface or underlying base therefor. At least one of the electrodes (on at least one side of the PTC element) is configured for maximum surface area in contact with the article being heated.
For example, the electrode on at least one side may have an extended flat surface if intended for use in heating articles with flat or nearly flat surfaces. Alternatively the surface may be that of a threaded fastener, if heating an article with a mating thread (FIGS. 2-6, 14, 17, and 18).
Ceramic elements are used instead of a polymer element because of their sharp temperature characteristic, as illustrated in FIG. 20. This characteristic describes a typical element with nominal 2,000 xcexa9 resistance at room temperature and transition temperature of 100xc2x0 C. At temperatures up to the transition temperature, resistance is substantially independent of temperature, though it falls slightly as temperature increases.
Once the transition temperature is reached, the element""s electrical resistance begins to rise rapidly, reducing power dissipation and generated heat, and limiting the attainable temperature. According to this typical PTC characteristic, by 110xc2x0 C. resistance has risen to 3,500 xcexa9 and power output is reduced from about 7xc2xd W to 4 W. By 120xc2x0 C., resistance is over 10,000 xcexa91 and power output down to about 1 watt. Thus, depending on the heat losses and capacity of the surface being heated, the heater will generally reach equilibrium slightly above 100xc2x0 C.
Soldering minimizes dimensions, and also minimizes thermal resistance between the element and the electrode, so that electrode temperature will generally be close to element temperature.
Soldering without risk of destructive thermal expansion on heating is made possible by any of several means:
1. Prestressing the copper components in tension and the ceramic element in compression. FIG. 3 illustrates an embodiment in which the surface to be heated is the threaded surface of a protruding bolt or stud. In this embodiment, the entirety of the copper components on both sides of the heating element are heated during soldering to above the melting point of the solder. Preferably the assembly consisting of the ceramic element and the copper components is heated up to soldering temperature through the copper, i.e. with heat applied from outside the assembly, through the copper to the junction of copper and ceramic element, while the assembly is pressed together. The assembly is then cooled relatively slowly back down through the melting point of solder.
By following this procedure, the copper is prestressed in tension and the ceramic element is prestressed in compression at room temperature, taking advantage of the strength characteristics of both components in the same manner as concrete prestressed with steel reinforcing bars. Upon heating, the residual stresses tend to disappear as the copper expands.
2. Utilizing numerous closely spaced deep grooves on the copper surface. FIG. 4 illustrates another embodiment directed to heating the threaded surface of a protruding bolt or stud, in which the copper surface to be soldered to the element includes numerous closely spaced deep grooves. The solder bond is effectively at the ends of stubby beams that can bend and shear relatively easily, to minimize stress on the element.
3. Utilizing a patch of copper screen. FIG. 5, showing another embodiment which is similarly directed to heating a threaded surface, illustrates a solder bond across a patch of copper screen, which serves a function similar to that of the deep grooves (described above in connection with FIG. 4).
Precautions such as those described above are necessary only because the element is being soldered to a substantial metal electrode. The electrode is thick relative to the element and is capable of exerting substantial force on the element absent such measures. If the metal form is relatively thin and small, such as electrical contacts with no mechanical or thermal function, then soldering is common without such precautionary measures.
Relatively heavy metal forms are necessary at least on one side, to assure maximum thermal contact with minimum resistance to heat transfer.
The soldered construction enables a number of novel configurations with performance advantages.
FIGS. 7-13 illustrate a prototype heater button which is a three piece soldered assembly. (FIG. 7 is an external view of a heater button and its internal details are hidden behind the electrical insulation which encapsulates the heater button. These internal details can best be seen in FIGS. 9, 10, 13.) This embodiment is directed to the objective of transferring heat through upper and/or lower flat heat transfer surface, to a compatible surface that is either flat or has very mild curvature. In the center of the prototype""s sandwich construction is a small PTC element, about xc2xcxe2x80x3xc3x97xc2xdxe2x80x3xc3x970.08xe2x80x3 (about 6 mmxc3x9712 mmxc3x972 mm) thick. The PTC element is soldered on both sides to copper discs that are thicker and larger than it is. Thus the heat transfer area, i.e. the area in contact with the surface being heated, is more than doubled from approximately 0.125 in (80 mm ) (area of the element) to 0.25 in (160 mm ) (area of a copper disc) with almost no temperature drop in the transition.
Electrical insulation on the copper disk may be an adhesively applied patch of dielectric film, with both the film and adhesive material suitable for elevated service temperature, such as polyimide film with silicone adhesive. The surface of this electrical insulation then is the heat transfer surface. Such film with adhesive is commercially available as tape with total thickness less than 0.003xe2x80x3, and can be readily cut to the shape of the surface of the copper disk. Alternatively and preferably, electrical insulation may be a thin film of heat resistant coating applied to the copper discs (FIG. 1). Thermal resistance is minimized, so that the surface being heated can indeed be heated to nearly element temperature. In this case the dielectric gap between the PTC heat-generating ceramic material and the heat transfer surface, which gap is comprised primarily of the electrical insulation, is less than 0.01xe2x80x3 (0.25 mm) thick over substantially the entire contact or heat transfer area. This is achieved in part by making the surface of the copper disk adequately smooth, and flat or otherwise conforming to the surface to be heated, and by then applying a thin layer of insulation, the surface of which then constitutes the heat transfer surface.
Referring to FIG. 2, the heater is externally insulated with heat shrink tubing. More critical is the manner of electrically insulating internally, from the bolt or stud being heated, because insulation between the male and female threads could seriously impede heat transfer.
Referring to FIGS. 3 through 6, electrical insulation is provided by a black anodize film, only about 0.0003xe2x80x3 (0.0076 mm) thick, on the threaded aluminum standoff/sleeve that is a mild shrink fit in the heater. (The dielectric film on the standoff/sleeve, although present, is too thin to be shown in these figures.) The film is then completely protected from mechanical damage, provides strong electrical insulation between the heater body and threaded sleeve, and provides negligible resistance to heat transfer. The combination of the threaded sleeve with oxide-film insulation, with the soldered bond between the heating element and electrode, makes the entire system possible in an extremely compact and practical package. The total dielectric gap between the PTC heat-generating ceramic material and the heat transfer surface, which gap in this case is comprised primarily of the black anodize film on the aluminum sleeve, is less than about 0.0006xe2x80x3 (0.015 mm) thick in total (including both internal and external sleeve surfaces), over substantially the entire contact or heat transfer area. This is achieved here in part by making the heat transfer surface conform to the surface to be heated.
Because of the relative precision with which temperature can be regulated, there will be numerous applications in process control and especially in the cure of high performance epoxy composite structures, particularly for the purpose of repair.
A heater tape constructed of multiple soldered PTC modules can be used to provide a warm boundary around an area to be heated through a precise temperature cycle, or to isolate the area from local heat sinks. This permits a dramatic improvement in temperature uniformity within the temperature controlled area.
In tests with this technique, temperature variation within a heated repair area was reduced, from more than xc2x120xc2x0 F. (xc2x111xc2x0 C.), to only xc2x18xc2x0 F. (xc2x14xc2x0 C.)(at the nominal control temperature of 210xc2x0 F. (99xc2x0 C.). Temperature uniformity was substantially improved over the entire heating and soak cycle (during which the surface was held, or xe2x80x9csoakedxe2x80x9d, at a predetermined constant temperature for a specific period of time required by the repair material process).
Prior to the above-described test, the same test was attempted with a purchased heater tape that was also nominally self-regulating according to the background art. Due to the combination of low power density, poor thermal coupling (excessive thermal resistance to heat transfer between the heater and the heated surface) and lack of a sharp transition temperature, the background art heater tape yielded no noticeable improvement over attempts to heat the repair area without any heater tape at all around the boundary.
In addition to the use of the improved PTC heaters as a boundary around a repair area heated by other means, heaters according to the present invention can also be used as the primary source of heat for temperature regulation over an entire repair area. However this is only appropriate where a single specific cure temperature is called for, rather than a cycle of varying temperatures.
Finally, the heaters can also be used in the bonding of studs to composite surfaces, also with high performance epoxy resins and particularly in aircraft maintenance. Cure time is reduced from several days to only one hour by safely heating to the regulated, elevated cure temperature with heaters such as illustrated in FIGS. 2 through 6, 14, and 18.
Summarizing:
Ceramic PTC heating elements are soldered to metal electrodes that are large and thick relative to the elements themselves, with provision to avoid destructive tensile stresses in the ceramic on heating, e.g.:
The elements are prestressed in compression at room temperature.
The metal surface at the soldered interface is divided by deep grooves into a series of stubby beams, with the element conductively bonded to the ends of these beams.
Solderable copper or brass electrodes are provided with an extended surface area compatible in shape and form with the surface to be heated.
Heat transfer surfaces are to be electrically insulated with a thin dielectric film which may be applied, for example, as a tape or coating. The surface of the dielectric film then becomes the new heat transfer surface.
Electrical insulation can also be obtained by transferring heat from the brass or copper electrode to an aluminum intermediate member over an extended heat transfer area, after coating the aluminum member with an electrically non-conductive anodize such as black anodize. Preferably, the solderable and aluminum members are contacted with a prestress, such as by shrink fitting the copper over the aluminum, so that the anodize coating is prestressed in compression at room temperature.
More new features are described in connection with Other Embodiments.
FIG. 8 shows a plan or top view of a heater tape constructed of multiple modules such as the heating button of FIG. 7. Among its features:
a. The modules are connected by a flexible skeletal member that may be, for example, a strip of silicone rubber and/or fiberglass, with holes to accommodate the connection of upper-to-lower portions of each module.
b. The skeletal member may be sharply necked down between modules, as illustrated in FIG. 8, to allow ease of bending in the side-to-side direction as well as up and down.
c. Upper electrodes of adjacent modules are connected by a running bus-wire, as are lower electrodes by another bus-wire, so that all the modules are electrically in parallel though mechanically in series.
d. The bus wires are bent between modules so that the total length of bus wire between two modules is sufficient to accommodate up-down bending of the tape with neutral axis at the skeletal member. This is further illustrated in FIG. 10 which shows a side view of the tape undergoing a 90xc2x0 downward bend. When the tape is bent in this manner, the bus wire closer to the center of curvature must become effectively shorter and the bus wire farther from the center of curvature must become effectively longer. The bends in the bus wire effectively store extra length of bus wire to accommodate the requirements of bending the tape.
e. The tape configured in this manner can bend up and down as well as sideways and can also be twisted in torsion to follow surface contours as necessary. A prototype tape of this type was used to provide a warm boundary about a repair area in a heavy section of graphite-reinforced epoxy. The tape provided an actual 120 watts/foot of heat to maintain temperature uniformity at the boundary as described above at startup, and, at equilibrium, more than 26 watts/foot.